Dormant viruses that most people carry for life without symptoms reactivated in nearly half of patients hospitalized with COVID-19, and that reactivation tracked with how sick those patients became, according to a large observational study published this week in Nature.
The finding does not mean COVID-19 causes new infections from outside the body. It means that viruses already living quietly inside patients, kept in check by a functioning immune system, started replicating again during severe illness. Most people carry roughly eight to 12 generally harmless viruses at any given time, and physiological stress can wake them.
For readers who had a severe COVID hospitalization or who are living with long COVID, the practical relevance is limited but real. The study offers a possible biological explanation for symptoms that have been difficult to account for, but it does not identify a treatment, a test to request or a change in medical care.
Measuring Active Replication, Not Antibodies
The research analyzed data from 1,154 adults hospitalized with COVID-19 at 20 U.S. hospitals between May 2020 and March 2021, drawn from the Immunophenotyping Assessment in a COVID-19 Cohort study. All participants were unvaccinated, and all were infected before newer variants became dominant.
Researchers used genomic sequencing on nasal swabs, blood cells and endotracheal aspirates from ventilated patients, generating more than a billion data points from roughly 200,000 samples, and followed participants during hospitalization and for up to 12 months afterward. According to CIDRAP's report on the study, the analysis was unusual in that it measured RNA from actively replicating viruses rather than relying on antibody responses.
That distinction is the study's methodological strength. Antibody tests can show that a person was once infected or that the immune system is reacting, but they cannot confirm a virus is currently replicating. Sequencing viral RNA gets closer to that question. Researchers reported that nearly half of participants had a virus reawaken, including in people whose immune systems were otherwise normal.
Which Viruses Woke Up and When
Eleven reactivated viruses were detected within the first 40 days after hospital admission. The most commonly detected were herpes simplex virus 1 and anelloviruses, a poorly understood family that establishes lifelong infection in roughly 90 percent of people, with Epstein-Barr virus, cytomegalovirus and enteroviruses found less often.
The viruses did not all reactivate at once. Epstein-Barr virus activity was most common around the time patients were admitted, detectable in about 24 percent of participants near admission before gradually declining, while cytomegalovirus and herpes simplex virus emerged roughly three weeks later, mainly in respiratory samples. That pattern suggests distinct triggers rather than a single collapse of immune control.
Two associations stood out. Reactivation of Epstein-Barr virus and cytomegalovirus correlated with systemic inflammation rather than with immunosuppression, which runs against the intuition that these viruses only emerge when the immune system is weakened. Anellovirus reactivation was associated with later physical disability and long COVID. Among severely ill patients, those with cytomegalovirus in the respiratory tract or Epstein-Barr virus in the nasal cavity were more likely to die within one year.
Where the Evidence Stops
This limitation belongs early rather than buried at the end. The study is observational, and the authors state plainly in the published paper that their results "do not establish causation between virus reactivation and clinical outcomes."
That leaves an unresolved question at the center of the finding. Reactivation may worsen outcomes, or severe illness may simply cause reactivation, or both may reflect a third factor such as intense systemic inflammation. The data cannot separate those explanations.
The population studied also limits how far the results travel. Every participant was unvaccinated and hospitalized during the first year of the pandemic, before Omicron-era variants and before widespread population immunity. Whether the same pattern occurs in vaccinated patients, in people with milder infections, or with current variants was not tested.
Researchers described the scale of the resource rather than any therapeutic claim. Joann Diray Arce, who leads the data management and analysis core at Boston Children's Hospital Precision Vaccines Program, said the work is the largest and most comprehensive biomarker study of COVID-19 conducted to date, according to the Boston Children's summary of the research.
Reasonable Next Steps for Patients and Families
No one should request antiviral treatment, Epstein-Barr virus testing or antiherpetic therapy on the basis of this study. There is no evidence yet that suppressing reactivated viruses improves outcomes, and antiviral drugs carry real risks including kidney effects and blood count suppression.
What the findings do support is continued attention to preventing severe COVID-19 in the first place, particularly for older adults, people who are immunocompromised and those with chronic heart, lung, kidney or metabolic conditions. Severity is the variable that tracked with reactivation.
People experiencing persistent fatigue, exercise intolerance, cognitive difficulty or new physical limitation after a COVID hospitalization should raise those symptoms with a clinician and ask about referral to a post-COVID or long COVID clinic, where available. Symptoms should be documented over time, including onset, duration and what makes them worse, since that record is more useful to a clinician than a single description.
Researchers say the immune, transcriptomic and metabolomic signatures identified here could eventually inform prognosis and treatment strategies. Whether that translates into anything a patient can use will depend on interventional trials that have not yet been conducted, and on replication in vaccinated populations and in current variant conditions.
The bottom line: the newest confirmed finding is that dormant viruses reactivated in nearly half of hospitalized COVID patients and tracked with severity and long COVID, the people most affected are those who were critically ill, and nothing in the study changes medical guidance today.
Frequently Asked Questions
What did the study find? Among 1,154 patients hospitalized with COVID-19, dormant viruses reactivated in nearly half, and that reactivation correlated with disease severity, inflammation and clinical outcomes.
Which viruses reactivated? Eleven were detected, most commonly herpes simplex virus 1 and anelloviruses, with Epstein-Barr virus, cytomegalovirus and enteroviruses found less often.
Does this prove the reactivated viruses caused severe COVID or long COVID? No. The authors state the study does not establish causation. Severe illness may cause reactivation rather than the reverse.
Does this apply to people who had mild COVID or who were vaccinated? The study cannot answer that. All participants were unvaccinated and hospitalized between May 2020 and March 2021, before newer variants.
Should patients ask for testing for Epstein-Barr virus or antiviral treatment? No. There is no evidence that testing or treating reactivated viruses improves outcomes, and antivirals carry meaningful risks.
What is an anellovirus? A poorly understood family of viruses that establishes lifelong infection in roughly 90 percent of people without normally causing illness. In this study, its reactivation was linked to later physical disability and long COVID.
What should someone with lingering symptoms after COVID do? Document symptoms over time and discuss them with a clinician, including whether referral to a post-COVID clinic is appropriate.
